1. Field of the Invention
The present invention relates to a camera system that functions to communicate data between a camera body and a photographing lens.
2. Description of Related Art
In a recent single lens reflex camera having an automatic focusing device, information peculiar to a photographing lens, such as open F-number data used in an automatic exposure function or an automatic focusing function, is sent as electrical signals from the photographing lens to a CPU (central processing unit) of a camera body.
Information peculiar to the photographing lens is stored in a lens ROM provided in the photographing lens. The photographing lens and the camera body are provided with electrical contacts on a lens mount and a body mount to transmit and receive signal information between the photographing lens and the camera body, respectively. The body CPU transmits and receives the signal information to and from the lens ROM through the electrical contacts and reads the data stored in the lens ROM. A clock pulse is outputted from the camera body to send address signals from the body CPU synchronously therewith, so that predetermined data can be output from the lens ROM in accordance with the address signals.
However, in an arrangement in which a motor is provided also in the photographing lens to control the power zooming etc., if all the control operations in the photographing lens are effected by the body CPU, there is an over load thereon. In particular, in a single lens reflex camera in which various kinds of photographing lenses are exchangeably mounted to the same camera body, the body CPU must read different parameters, depending on the photographing lenses from the associated lens ROM's and perform predetermined arithmetic operations in accordance with the read parameters thereby to control the photographing lenses. This makes quick operations impossible or next to impossible.
In a zoom lens, a minimum Av value (open diaphragm F number) increases as the focal length increases. Namely, the relationship between the minimum Av value and a focal length is represented by a linear function. Accordingly, the minimum Av values corresponding to the focal lengths are all stored in the ROM of the photographing lens. Consequently, various data, including the minimum Av values are stored in a ROM so that the memory (capacity) of the ROM, in which the data can be written is reduced. Therefore, the stored data must be partially erased, or a large ROM must be used instead of a conventional ROM to write new data.